Glacial Modification of Terrain

Types of Glaciers

  • Formed by the accumulation and compaction of recrystallized snow.
  • Ice mass motion under gravity grinds anything in its path and deposits rock in a new location, changing the landscape.
  • Glaciation increases the erosion rate on a mountain by at least 10x compared to an unglaciated mountain.
  • It modifies flat landscapes as well.
  • Ice must be moving or flowing.
  • The pattern of movement and effect on topography vary.
  • Glacial processes produce a combination of constructive and destructive glacial landforms.

Mountain Glaciers

  • High altitude polar environments.
  • Alpine glaciers or "Valley Glaciers".

Continental Ice Sheets

  • High latitude polar environments.
  • Ice Sheets or "Continental Glaciers".

Continental Ice Sheets

  • Glaciers that form in non-mountainous areas of the continents.
  • High latitude polar environments.
  • Ice Sheets or "Continental Glaciers".
  • Cover extensive areas of continental landmasses.
  • Long periods of extremely low temperatures.
  • Antarctica (90%) and Greenland (80%) are almost completely covered by ice sheets.
  • Outlet Glacier - a valley glacier which drains an inland ice sheet or ice cap and flows through a gap in peripheral mountains.
    • Long "tongues" of ice
    • Extend between rimming hills to the sea
  • Ice Shelf - portion of an ice sheet that spreads out over water.
  • Calving - chunks of ice breaking off from ice shelves and outlet glaciers and falling into the sea.
  • Ice masses float away as icebergs.

Mountain Glaciers

  • A glacier that is confined by surrounding mountain terrain; also called an alpine glacier.
  • High altitude polar environments.
  • Alpine Glaciers or "Valley Glaciers".
  • Long, linear glaciers that occupy high-altitude mountain valleys.
  • Flow down the valley, and increase in size as they accumulate and absorb smaller tributary glaciers from the mountainous terrain.
  • Highland icefields that overlay all the underlying topography.
  • Found throughout the world: Rockies, Andes, and Himalayas.
  • High-latitude, polar or arctic mountains, such as those in Alaska.
  • Protuding pinnalces - nunatak.
  • Outlets tongues of ice that travel down mountain valleys (valley glacier).
  • If the leading edge of a valley glacier reaches a flat area and escapes from the confines of its valley walls (piedmont glacier).
  • Valley and piedmont glaciers originate in the high alpine and terminate on land.
  • Alpine glaciers - Develop individually instead of part of a broad ice field.
  • Cirque glaciers - small alpine glaciers that are confined to the basins where they originated; their basins are called cirques (landform feature).

Glaciations Past & Present

  • Glacial ice volume has varied considerably over the last few million years.
  • Evidence left behind allows scientists to determine the chronology of past glaciations.

Glaciation: Pleistocene

  • Began at least 2.59 million years ago.
  • Last major ice retreat occurred only 9000 years ago.
  • The dominant environmental characteristic was the refrigeration of high-latitude and high- elevation areas.
  • Consistent alterations of glacial and interglacial periods.
  • Wisconsin glacial stage marked the end.
  • Coincided with the conclusion of what is known as North America.
  • Period since then: Holocene Epoch.
    • Either a post-glacial period or the latest in a series of interglacial interludes.
  • At its peak, one-third of the total land was covered in ice.
  • Laurentide Ice was the most extensive ice mass.
  • The Driftless Area is a small area in Wisconsin that was never completely surrounded by ice.

Glaciation: Periglacial Process

  • Beyond the extent of ice advance.
  • Periglacial zone – a zone where ice never existed but glacial factors affected the landscape, such as erosion from ice melt.
  • Important Processes:
    • Erosion and Deposition
    • Frost weathering
    • Solifuction of frozen
  • Extended over more than 20% of Earth's land area.
  • INDIRECT EFFECTS.

Glaciation: Sea-Level Changes

  • Build-up of ice on continents led to less drain water on continents and brought about a lowering of sea levels up to 100-meter drop in sea level worldwide.
  • Coastal plains become extensive.
  • Continental islands disappeared.
  • Land bridges exposed.
  • INDIRECT EFFECTS.

Glaciation: Crustal Depression

  • The weight of the ice on the continents caused continents to sink.
  • Ice melt allowed for continental rebound.
  • INDIRECT EFFECTS.

Glaciation: Pluvial Developments

  • Considerable runoff results in increased moisture, leading to increased precipitation and less evaporation.
  • Developed many lakes, including the Great Salt Lake (formed from Lake Bonneville).
  • INDIRECT EFFECTS.

Glaciation: Contemporary Glaciation

  • Limited ice cover today (about 10 percent of total land surface).
  • 96 percent of the total ice cover is Greenland and Antarctica.

Glaciation: Antarctic Ice Sheet

  • Consists of two unequal sections separated by Transantarctic Mountains.
  • West Antarctica has a few “dry valleys” - winds blast away snow and keep precipitation out (doesn't build ice).

Glaciation: Greenland Ice Sheet

  • Less extensive than Antarctica.
  • Relatively small ice masses on certain islands in the Canadian Arctic, Iceland, and some of islands north of Europe.

Glaciation: North American Glaciers

  • The remainder of present-day glaciers are concentrated in high mountain areas.
  • US: PNW (North Cascade Mountains, WA, and Alaska).

Glaciation: Climate Change

  • Retreating of polar ice caps.
  • Shrinking ice caps are an indicator of a warming climate.
  • Antarctic ice shelves breaking.
  • Higher flow rates of outlet glaciers.

Formation: Glacier Formation and Movement

  • Requires a balance between accumulation and ablation.
  • Snow begins as crystallized water vapor.
  • Compressed to granular form.
  • Density doubles.
  • More compression causes granule to coalesce, névé/firn half density of water.
  • Further compression results in glacial ice.
  • Density ~90% of liquid water.
  • Glaciers take on a bluish tint because dense glacial ice absorbs most wavelengths of visible light.
  • Reflects and scatters blue light.
  • Glacier “flow” is the orderly sliding of ice molecules.
  • Ice under extreme pressure deforms instead of slipping.
  • Meltwater contributes surface for glaciers to slide on.
  • Flow in response to overlying weight.

Movement: Plastic Flow of Ice

  • Response to the overlying weight.
  • Occurs at ~50 meters thick.
  • Ice oozes outward from around the edge of an ice sheet or down the valley from the end of the glacier.

Movement: Basal Slip

  • At the bottom of the glacier
  • The entire mass slides over its bed on a lubricating film of water.
  • The glacier molds itself to the shape of the terrain.

Movement: Rates of Movement

  • Exceedingly slow and erratic.
  • The fastest-moving ice is near the surface; the Center moves faster than the sides.

Flow vs. Advance

  • Glaciers are always flowing downward or laterally, not necessarily advancing.
  • Depends on the balance between accumulation and ablation.
  • Even in retreating glaciers (heavy ablation), the ice is flowing forward.
  • During wetter/cooler periods of great accumulation, glaciers flow farther.
    • Causes the outer margin to advance
  • During warmer/drier periods of great ablation, glaciers waste away sooner, so the end (or terminus).
    • Causes the glacier to retreat

Effects of Glaciers

  • Volume and speed determine the success of glacial erosion.
  • The erosive power of moving ice is slightly larger than that of water.
  • Glacial Plucking.
  • Glacial Abrasion.
  • Subglacial meltwater erosion.

Erosion by Glaciers: Glacial Plucking

  • Picking up rock material through refreezing of meltwater.
  • Freeze/thaw action of the ice breaks down the bedrock (hydraulic wedging).
  • Mechanical Erosion.
  • Over time, these rocks become loose and become trapped in the glacier.

Erosion by Glaciers: Glacial Abrasion

  • Bedrock worn down by rock debris embedded in glacier.
  • The ice at the bottom of a glacier usually has bits of rock, sediment, and debris, creating a rough sandpaper texture.
  • As a glacier flows downslope, it drags the rock, sediment, and debris in its basal ice over the bedrock beneath it, grinding it.
  • Produces scratches (striations) in the bedrock surface.

Erosion by Glaciers: Subglacial Meltwater Erosion

  • Meltwater streams flowing beneath the glacier transport rock, and erode smooth grooves and channels into the bedrock.
  • Glacial Striations: As ancient glaciers flowed over basalt. rock and sediment in the ice left scratches on the bedrock. These scratches, "striations," can be used to understand past ice flow. This rock has been scratched so much it shines with "glacier polish."
  • Valuable tool when reconstructing past glaciers.

Effects of Glaciers: Transportation by glaciers

  • Glaciers are effective in moving large rock pieces.
  • Most rock material is transported along the base of the ice.
  • Typically move glacial flour.

Transportation by Glaciers: Glacier flour

  • Glacier sediment that is much finer than sand.
  • Easily transported by and suspended in water.
  • Creates a cloudy or milky appearance of streams, rivers, and lakes that are fed by glaciers.
  • Responsible for the turquoise color in glacial lakes.
  • The more glacial flour present in the waters, the greener they will appear to our eyes!

Effects of Glaciers: Transportation by glaciers

  • Remaining glacial ice free of rock debris.
  • The role of flowing water on moving ice, melt streams.
  • Cracks in ice in which streams run – moulins (melt streams).

Effects of Glaciers: Deposition by Glaciers

  • Glaciers move lithospheric material from one region to another in a vastly different form.
  • Material moved by glaciers – drift.
  • Till – rock debris deposited by moving or melting ice.
  • Large boulders that are different from surrounding local bedrock, glacial erratics.

Effects of Glaciers: Deposition by Meltwater

  • A large portion of debris carried by glaciers deposited or redeposited by meltwater.
  • Subglacial streams from glaciers carry sedimentary material.
  • Glaciofluvial deposition.

Continental Ice Sheets

  • The third most extensive feature on the planet.
  • Development and flow of ice sheets.
  • Pleistocene ice sheets originated in midlatitudes and subpolar regions.
  • Ice flowed outward from the center of accumulation.
  • Ice sheets ebbed and flowed with changing climate.

Continental Ice Sheets: Erosion by Ice Sheets

  • The principal topography from the ice sheet is a gently undulating surface.
  • Valley bottoms created from moving ice.
  • Roche mountonnée, stoss side versus lee side.
  • A postglacial landscape has low relief but is not absolutely flat.

Erosion by Ice Sheets: Rouche Moutonnée

  • Produced when a bedrock hill is overridden by moving ice.
  • Stoss side - smoothly rounded and streamlined by grinding abrasion as the ice rides up the slope.
  • Lee side - shaped by plucking, which produces a steeper, more irregular shape.

Deposition by Ice Sheets: Moraines

  • Land consisting primarily of till.
  • Three types of moraines:
    • Terminal moraine – marks the outermost limit of glacial advance
    • Recessional moraine – positions where the ice front is stabilized
    • Ground moraine – large quantities of till laid down from under a glacier instead of from its edge, kettles

Deposition by Mountain Glaciers: Terminal & Recessional Moraines

  • Usually built from rocks and debris that are transported to the glacier toe in the ice and melt out there.
  • If the glacier terminus stays in one position for a long time, more debris will accumulate there, building a larger terminal moraine.
  • Can be preserved for thousands of years if the glacier does not readvance
  • A quick glacial retreat will cause its terminal position to change every few years.
    • Creates a series of smaller recessional moraines

Deposition by Ice Sheets: Kettles

  • As a glacier recedes, sediment is washed out from the glacier and deposited in a flat area below, forming an outwash plain.
  • Depressions, known as kettles, often pockmark these outwash plains and other areas with glacial deposits.
  • Kettles form when a block of stagnant ice (a serac) detaches from the glacier.
  • Eventually, it becomes wholly or partially buried in sediment and slowly melts, leaving behind a pit.
  • In many cases, water begins to fill the depression and forms a shallow pond or lake (kettle).

Deposition by Ice Sheets: Drumlins

  • Hills of sediment (generally a quarter of a mile or more in length) that have been streamlined by glacier flow.
  • Characteristics:
    • Elongated, longest in the direction of flow
    • Often occur together in fields
    • Classic shape: half-buried egg
  • Scientists often use them to understand past glacier flow directions.

Glaciofluvial Features

  • Deposition of debris by ice-sheet meltwater produces features; composed of stratified drift.
  • Composed of gravel, sand, silt since meltwater is incapable of moving larger material.
  • OUTWASH PLAINS.
  • VALLEY TRAINS.
  • ESKERS.
  • KAMES.
  • LAKES VERY COMMON.

Glaciofluvial Features: Outwash Plains

  • Occur in front of melting glaciers.
  • Expansive, generally flat areas.
  • Dominated by braided rivers when the glacier is actively melting.
  • In areas that were once glaciated, old outwash plains can be found by looking for glacial sediment (till) that has been sorted by grain or boulder size as it is picked up and deposited by flowing water.
  • Sediment is typically finest farthest away from the glacier.
  • Outwash plains can extend for miles beyond the glacier margin.

Glaciofluvial Features: Eskers

  • Meandering ridges of sediment that form in water channels beneath or within the glacier ice.
  • The floors of these channels can be rock, sediment, or ice.
  • As water speeds in the channels slow (during retreat or during periods of low melt), it drops the sediment it carries and builds small piles that take on the shape of the channels.

Glaciofluvial Features: Kames

  • Deposited mound of sediment left in the path of a retreating glacier
  • Rocks/sediment fall into crevasses in the glacier
  • Deglaciation - crevasses move closer to the base as the glacier melts
  • When they reach the valley bed, they are deposition onto the bed as a mound of sediment

Glaciofluvial Features: Lakes

  • Very common in areas that were glaciated during the Pleistocene.
  • Compare the northern and southern parts of the U.S.
  • Great Lakes Formed as a result of both glacial erosion and deposition during Plesitocene
  • The result of repeated advances and retreats of ice sheets

Mountain Glaciers

  • Development and Flow Usually form in sheltered depressions near heads of stream valleys
  • Erosion by Mountain Glaciers Basic landform in glaciated mountains is the cirque
  • Marks the location where an alpine glacier originated

Erosion by Mountain Glaciers: Cirques

  • Bowl-shaped, amphitheater-like depressions that glaciers carve into mountains and valley sidewalls at high elevations.
  • Often, the glaciers flow up and over the lip of the cirque as gravity drives them downslope.
  • Lakes (called tarns) often occupy these depressions once the glaciers retreat.

Erosion by Mountain Glaciers: Bergshrund

  • Shifting equilibrium line generates quarrying action, bergschrund formation.
  • Quarried fragments from cirque are carried away when ice flows out of the cirque.
  • Cirque ice melts away; the depression that holds water is a tarn.
  • Often dammed by moraines.

Glaciofluvial Features: Tarns

  • In active glaciers, tarns are full of glacially ground sediment that scatters light and can make the water appear colorful.
  • The color of a glacier's tarn is a good way to diagnose whether or not the ice is still actively moving.
  • Crevasses on the glacier surface are another good indication of glacier movement, too.

Erosion by Mountain Glaciers: Arêtes

  • Several cirques cut back into interfluve result in the spine of rock, an arête.
  • A thin, jagged crest that separates—or that once separated—two adjacent glaciers.
  • Look like serrated knives or saw blades, with steep sides and a sharp crest.

Erosion by Mountain Glaciers: Cols

  • The low points on the serrated surface are known as cols.
  • Cols act as spillways for the ice and occur where glacier action has eroded the rock sufficiently to overtop it.

Erosion by Mountain Glaciers: Horns

  • Horns are pointed peaks that are bounded on at least three sides by glaciers.
  • They typically have flat faces that give them a somewhat pyramidal shape and sharp, distinct edges.

Erosion by Mountain Glaciers: Erosion in the Valleys

  • Some glaciers never leave cirques.
  • The principle erosive work is to deepen, steepen, and widen the valley.
  • U-shaped glacial troughs.

Erosion by Mountain Glaciers: Glacial Troughs

  • Glaciers flow downslope, taking the easiest path (v-shaped valleys carved by rivers).
  • As glaciers flow through these valleys, they concentrate erosive action over the entire valley, widening its floor and over-steepening its walls.
  • After the glacier retreats, it leaves behind a flat-bottomed, steep-walled U-shaped valley.

Erosion by Mountain Glaciers: Glacial Steps

  • Glacial steps result from differences in rock resistance.
  • Small lakes may remain in a shallow depression on the benches of the glacial steps, forming paternoster lakes.

Erosion by Mountain Glaciers: Hanging Valleys (Glacial Troughs)

  • A former tributary glacier valley that is incised into the upper part of a U-shaped glacier valley, higher than the floor of the main valley.
  • Hanging valley streams often enter the main valley as waterfalls.

Deposition by Mountain Glaciers: Deposition by Mountain Glaciers

  • Continental ice sheets are more responsible for deposition than mountain glaciation.
  • Moraines are the primary deposition mechanism.
    • Lateral moraines.
    • Medial morraines.

Deposition by Mountain Glaciers: Lateral and medial moraines

  • Consist of glacially-transported rock and debris.
  • They form on the sides of glaciers (lateral moraines) or at the boundary between two tributary glaciers (medial moraines).
  • Either way, they often mark the edges of an ice body.

Deposition by Mountain Glaciers: Lateral Morains

  • Sharp-crested piles of glacially-transported rocks and debris are dropped by the ice as it melts.
  • Form only in the ablation zone of a glacier.
  • Good indicators of where the equilibrium line occurred on past glaciers.
  • Remain on the landscape long after glacier retreat and are frequently contiguous with terminal moraines.

Deposition by Mountain Glaciers: Medial Morains

  • Medial moraines form where two tributary glaciers come together.
  • Generally surficial features on the ice and often consist of rock that has fallen from a rockwall where the glaciers converge.
  • Because they are thin, surficial features, medial moraines are rarely preserved after the ice retreats.

Periglacial Environment

  • Periglacial – on the perimeter of glaciation.
  • Permafrost presence.
  • Frozen ground exists in Alaska, Canada, and Russia.
  • Extends to great depths.
  • Patterned ground.
  • Proglacial lakes.

Periglacial Environment: Patterned Ground

  • Various geometric patterns that repeatedly appear over larger areas in the arctic.
  • The freeze-thaw cycle disrupts the uniform surface of soil and regolith.
  • Form rough polygons.
  • Ice wedges develop large polygonal patterns.

Periglacial Environment: Proglacial Lakes

  • Proglacial lakes are masses of water impounded at the edge of a glacier or at the margin of an ice sheet.

Causes of the Pleistocene Glaciations

  • What initiates ice ages?
  • Any plausible theory must account for four main characteristics:
    • Ice accumulation is in both hemispheres but nonuniform.
    • Concurrent development of pluvial conditions in dryland areas.
    • Multiple ice advance and retreat cycles.
    • Eventual total deglaciation.

Causes: Cold versus warm climate for glaciation

  • Colder conditions
    • Inhibit summer wastage
    • Enhance the longevity of the winter accumulation
  • Cold air cannot hold much water vapor
  • Warmer winters would favor increased snowfall
  • Cooler summers are needed for decreased melting

Causes: Milankovitch Cycles

  • Variations in inclination, eccentricity, and precision of the equinoxes.
  • Correlates well with some (but not all) of the major glacial advances and retreats during the Pleistocene.

Causes: Other Climatic Factors

  • Variations in solar output.
  • Variations in carbon dioxide in the atmosphere.
  • Changes in continental positions.
  • Atmospheric circulations.
  • Tectonic upheaval